The present invention relates to flow control valves, and more particularly, to such valves which are provided with a "Float" position.
More specifically, the present invention relates to a spool valve which is able to control the flow of fluid from a source, such as a pump to a work port, wherein the flow to the work port is through a pilot operated check valve, and the invention will be described in connection therewith.
Many flow control valves of the type to which the present invention relates are utilized to control the flow of pressurized fluid from a source to a work implement, such as a cylinder, which is used in association with a piece of equipment which, under certain operating conditions, is allowed to "float". An example of such a system would be the mower deck on a large, commercial turf maintenance vehicle. The mower deck is mounted such that its mowing height is controlled by a cylinder, of which the upper chamber is vented to air, and the lower chamber is connected to a three way, three-position flow control spool valve. Typically, the spool valve is provided with three operating positions, including (1) Raise; (2) Neutral; and (3) Lower or Float.
As is well known to those skilled in the art, the Float position of the spool valve is one in which the inlet port (connected to the pump), the work port (connected to the cylinder lower chamber), and the return port (connected to tank) are all interconnected, thus permitting the implement (e.g., the mower deck), to have some vertical movement (i.e., to float) in response to external loads imposed upon the implement.
Many such flow control valves have been provided with pilot operated load holding check valves, the function of which, as is well known to those skilled in the art, is to prevent flow from the lower chamber of the cylinder back through the work port when the spool valve is in the Neutral position, and there is a load on the cylinder. Typically, the load holding check valve has been operably associated with a lockout plunger, the function of which is to hold the check valve open, thus permitting lowering of the cylinder, when high pressure is exerted on the plunger.
Although the above-described flow control valve has been generally satisfactory, the sequence of steps for lowering the cylinder has not been fully satisfactory. In order to lower the load with the prior art flow control valve, the operator would first move the spool to the Lower position, thus permitting lowering of the load, and then when the load was at approximately the desired position, the operator would move the spool to the Raise position. The movement of the spool to the Raise position was required because the lockout plunger was typically equipped with a detent which, after the plunger was subjected to high pressure, was held in the position corresponding to the open position of the check valve. Therefore, it was necessary to move the spool to the Raise position to exert high pressure on the lockout plunger, but now in the opposite direction, to move it out of engagement with the detent and back to its normal position. This movement of the lockout plunger would permit the check valve to close, thus cutting off the return flow of fluid from the implement. The operator would then move the spool to the Neutral position, thus maintaining the load in whatever position it occupied at that time.
Although the above sequence of steps resulted in generally satisfactory operation, the particular sequence of steps would not seem natural to the operator, and in an emergency, when the operator may not have the opportunity to think through the required sequence, it is possible that the operator will not provide input movement to the valve spool which is appropriate for that particular situation.